The present invention relates in general to pneumatic controllers, and in particular to a new and useful servo which has reduced susceptibility to oscillations.
U.S. Pat. No. 4,610,263, the disclosure of which is incorporated herein by reference, discloses a pneumatic servo assembly which utilizes a motor driven cam that moves in front of a pneumatic nozzle to cause a back pressure in the nozzle which is conveyed to a bellows as a pneumatic signal. The pneumatic signal is converted to an electrical signal and causes rotation of the cam to equalize the setting of the servo.
I/P positioners, such as the Type AP7 positioner available from Bailey Controls Company, are known which utilize similar servos. Servos, as described herein, are current-to-pneumatic converters that generally convert an electrical signal of 4 to 20 mA to a pressure signal of 3 to 15 psig.
FIG. 1 illustrates a servo of the prior art which comprises a motor 10 having a shaft with hub 12 which drives a cam 14. The outer periphery of cam 14 is profiled so that as hub 12 rotates, the outlet opening of a nozzle 16 is either fully covered, fully exposed or partially covered, to produce a back pressure in an air hose 18 connected to the nozzle. A nozzle support or bracket 20 having a bore therethrough for receiving nozzle 16, holds the nozzle in a fixed position with a set or lock screw 22. Bracket 20 includes a flange 24 with an upper surface that backs up the cam 14. Flange 24 is positioned below nozzle 16 with cam 14 sliding between the nozzle and the flange. Flange 24 includes an air exhaust hole 26 which cooperates with air leaving nozzle 16 and the cross-sectional area of cam 14 covering the nozzle, to accurately produce a back pressure signal in hose 18.
As shown in FIG. 1, electrical signals can be provided over a pair of electrical lines 28 to motor 10 to rotate hub 12 and cam 14. As a result of its rotation, cam 14 in turn presents a different portion of its profiled outer periphery to the nozzle opening of nozzle 16, to change the pneumatic signal in the form of back pressure in the hose 18.
It has been found that servos of the design shown on FIG. 1, are not always stable above 12 psi. It has been found that such servos oscillate with excursions of 0.7% to 0.8% at constant input to the servo.